Methane production by anaerobic digestion of plant material and organic waste

ABSTRACT

A process for production of methane gas in higher yields and higher rates by thermophilic or mesophilic anaerobic digestion of plant material and/or organic waste in admixture with an extract of different plant material. Increases of methane yield of greater than about 25% and up to about 500% are obtained by addition of the extract of different plant material to a normally low biodegradable plant and/or organic waste material before anaerobic digestion thereof. The resulting methane yields and production rates are higher than those obtained by the sum from anaerobic digestion of the individual components when using the extract as taught by this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Methane production of anaerobic digestion has been widely practiced,particularly with respect to digestion of sewage sludge organic waste.In recent times, the world-wide energy shortage has furtheredconsideration and improvement of such non-fossil sources of energy. Thisinvention relates to a process for improved methane production from andbeneficiation of anaerobic digestion of plant material and/or organicwaste comprising anaerobic digestion of plant material and/or organicwaste of normally low biodegradability in the presence of extract ofdifferent plant material. The extract is present in about 10 to about 90volume percent of the digester contents. The process may be carried outunder mesophilic or thermophilic temperatures for detention times inexcess of about four days. Under steady state anaerobic digestion, theplant material and/or organic waste of normally low biodegradability inthe presence of the extract of different plant material results insynergistic action providing higher methane yields and production ratesthan those that result from the anaerobic digestion of the individualfeed components separately.

2. Description of the Prior Art

The production of methane gas by anaerobic digestion of various organicwastes has been known. There have been continuous efforts to improvemethane yield resulting from anaerobic digestion. Most of the priorattempts to increase methane yield have been centered around anaerobicdigestion as practiced in municipal waste treatment plants asexemplified by U.S. Pat. Nos. 3,640,846, teaching addition of coal;3,981,800, teaching pressurized digestion; and 4,022,665, teaching twophase digestion of sewage sludge. Other attempts to improve theproduction rate and yield of methane by anaerobic digestion have relatedto improved anaerobic digestion by utilization of liberated enzymes ofthe biomass for contribution to more efficient digestion as taught byU.S. Pat. No. 3,994,780. The U.S. Pat. No. 3,994,780 patent teaches theapplicability of its process to a wide variety of organic feeds, butdoes not suggest the synergistically improved methane production byanaerobic digestion of plant material and/or organic waste of normallylow biodegradability in the presence of an extract of different plantmaterial. The anaerobic digestion of terrestrial plant material toproduce methane has been recognized as exemplified by D. L. Klass and S.Ghosh, "Methane Production by Anaerobic Digestion of Bermuda Grass",presented at symposium on Biomass as a Non-fossil Fuel Source, ACS/Chem.Soc. of Japan Joint Chemical Congress, Honolulu, Hawaii, Apr. 1-6, 1979.Likewise, the anaerobic digestion of aquatic plant material to producemethane has been recognized as exemplified by R. P. Lecuyer and J. H.Marten, "An Economic Assessment of Fuel Gas from Water Hyacinths",Symposium Papers, Clean Fuels from Biomass, Sewage, Urban Refuse,Agricultural Wastes, Orlando, Fla., Jan. 27-30, 1976. Again, thesynergism resulting in improved methane production by anaerobicdigestion of plant material and/or organic waste or normally lowbiodegradability in the presence of an extract of different plantmaterial is not suggested. Stimulation of methane production inanaerobic waste treatment by metal cations has been recognized as hasthe problem of toxicity in methane producing anaerobic systems asexemplified by I. J. Kugelman and K. K. Chin, "Toxicity, Synergism andAntagonism in Anaerobic Waste Treatment Processes", Anaerobic BiologicalTreatment Processes, Advances in Chemistry, Series 105 (1971). Whilerecognizing the sensitivity of methane production in anaerobic wastetreatment processes, there is no suggestion of the improvement ofanaerobic digestion of plant material and/or organic waste of normallylow biodegradability in the presence of an extract of different plantmaterial in methane production. In the past, there have been attempts toincrease methane production by the expensive addition of specificchemicals to the digester to overcome various deficiencies.

SUMMARY OF THE INVENTION

The process of this invention provides production of methane gas inhigher yields and higher rates by thermophilic or mesophilic anaerobicdigestion of plant material and/or organic waste of normally lowbiodegradability in the presence of an extract of different plantmaterial. The resulting methane yields and production rates are higherthan those obtained by the sum from anaerobic digestion of theindividual feed components. The plant material may be of terrestrial oraquatic origin. It is particularly preferred that the plant material bea mixture of terrestrial and aquatic plant materials. The practice ofthis invention utilizes natural materials without the need of expensiveisolation techniques or use of chemicals, per se.

The term "plant material" as used in this description and the appendedclaims includes any of the organisms of the kingdom of Plantae whichtypically have cell walls composed of cellulose in large part and havenutritive systems in which carbohydrates are formed photosynthetically.The plant material useful in this invention is fresh harvested or storedplant material, which is usually grown on farms for this purpose, and isuntreated chemically or physically, except for size reduction. Includedare both terrestrial plants and aquatic plants. Terrestrial plantsinclude warm season grasses, such as Bermuda grass and Elephant grass;cool season grasses, such as Kentucky Blue grass and Merion Blue grass;reedy plants, such as Bamboo, rice, cattails, herbaceous plants, such asKudzu and maze; deciduous trees, such as eucalyptus and poplar; andconiferous trees, such as white and red pines. Exemplary aquatic plantsinclude water hyacinth, duck weed, algae, sea kelp and sargassum.

Normally low biodegradable plants are those which are recalcitrant togas, particularly methane, production under conventional anaerobicdigestion conditions resulting in methane production of less than about2 SCF/lb. VS added. Included among preferred normally low biodegradableplants are high cellulosics such as Bermuda grass, bamboo, Kentucky bluegrass, pine trees, poplar trees, eucalyptus, cattails and mixturesthereof.

By the term "organic waste" as used in this disclosure and the appendedclaims, we mean all types of organic refuse including sewage sludge,animal waste, municipal waste, industrial waste, forestry waste,agricultural waste, and the like. By forestry waste and agriculturalwaste we mean to include portions of plants after some physical orchemical treatment, usually not including the entire plant, for example,stumps from logging, sawdust, wood chips, corn stalks, corncob andbagasse. Treatment of municipal solid waste for removal of undesiredmaterial such as glass, metals, plastics, stones, and the like, is wellknown to the art. Included among preferred normally low biodegradablewaste are cornstalks and municipal solid waste and mixtures thereof.

It is suitable for the anaerobic digester to include a slurry of plantmaterial and/or organic waste of normally low biodegradability in thepresence of an extract of different plant material. The extractcomprises about 10 to about 90 volume percent of the total digestionvolume, the remainder being water and plant and/or organic waste ofnormally low biodegradability. Particularly preferred are extractvolumes comprising about 15 to about 50 volume percent of digestionvolume. The extract is preferably liquid obtained from physical cuttingand/or pressing, crushing, steaming, water or solvent extraction andalkali or acid extraction with neutralization. Such methods are wellknown to one skilled in the art. The extract may be conveniently addedto the water used to form a slurry of the normally low biodegradableplant material and/or organic waste. The liquid extract reduces theamount of water required for the slurry. Mixtures of individual plantmaterials as defined above may be used in both the extract and in thenormally low biodegradable fraction. Particularly preferred are mixturesof terrestrial and aquatic plant materials. Preferred extracts arederived from water hyacinth, giant brown kelp, Chlorella, alfalfa, soybean plants and mixtures thereof. The extract may additionally compriseup to about 10 volume percent sludge liquor, preferably both activatedand primary. Preferred amounts of sludge liquor are about 1 to about 5volume percent, based upon the total digestion volume.

During anaerobic digestion, gas production is principally from the plantand/or organic waste material of normally low biodegradability, only aminor portion, generally on the order of less than 10 percent, of thetotal gas produced being derived from the extract, even at the higherranges of volumes of extract set forth above.

Known techniques may be used for anaerobic digestion of the plant and/ororganic waste material of normally low biodegradability in the presenceof extract of a different plant material. Detention times of in excessof 4 days and preferably about 8 to about 30 days are suitable.Detention times of about 11 to about 16 days are especially preferred.Increases of methane yield of greater than about 25 percent and up toabout 500 percent are obtained by addition of extract of different plantmaterial to a normally low biodegradable plant and/or organic wastematerial for anaerobic digestion. Methane production by anaerobicdigestion according to the process of this invention can be continuedfor long periods of time without addition of external nutrient. Methaneproduction is stable over long periods of digestion. Plant materials,for example herbacious plants such as Giant Reed, bamboo and grasses andwoody plants, such as Black Alder, Loblolly Pine, Eucalyptus and BoxElder and organic wastes, such as municipal solid waste and cornstalks,which are recalcitrant to anaerobic digestion alone are readily digestedusing the process of this invention involving addition of extract of adifferent plant material. The effluent from the anaerobic digestionaccording to the process of this invention contains a reduced amount ofunconverted residue and has a low concentration of soluble organicsindicating low ultimate disposal cost and the feasibility of its recycleto the anaerobic digester with little or no treatment. The digestedeffluent, although dilute, can be dewatered directly by vacuumfiltration or other dewatering method, such as settling or chemicalmethods, to provide cake-solids content and cake yield comparable tothat of filtered, digested sewage sludge.

It is an object of this invention to provide a process for methaneproduction resulting in higher yields and higher production rates thanpreviously obtained by anaerobic digestion of plant and/or organic wastematerial of normally low biodegradability.

It is another object of this invention to provide a process for methaneproduction by anaerobic digestion of plant and/or organic waste materialof normally low biodegradability which does not require addition ofexternal nutrient throughout the process.

It is yet another object of this invention to provide a process formethane production by anaerobic digestion resulting in digester effluentwhich can be easily dewatered and contains reduced amounts ofunconverted residue.

It is still another object of this invention to provide a process formethane production by anaerobic digestion resulting in the digestereffluent having low concentration of soluble organics providing easydisposal and recycling to the digester with little or no treatment.

It is another object of this invention to provide a process for methaneproduction from plant and/or organic waste material which is, by itself,recalcitrant to anaerobic digestion.

It is another object of this invention to provide a process suitable forproduction of synthetic natural gas (SNG) by an anaerobic digestionprocess comprising anaerobic digestion of plant and/or organic wastematerial of normally low biodegradability in the presence of an extractof different plant material, thereby allowing better matching of feedsupplies for continuous year round operation.

Yet another object of this invention is to provide an organic wasteutilizing methane production plant providing simultaneous energyrecovery and waste stabilization.

These and other objects and advantages are achieved by the process ofthis invention as set forth in the more detailed description ofpreferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The anaerobic digestion of plant and/or organic waste material ofnormally low biodegradability in the presence of an extract of differentplant material according to this invention, may be carried out underconditions of temperature, both mesophilic (about 20° to 45° C.) andthermophilic (about 45° to 70° C.); detention times in excess of about 4days and usually about 8 to 30 days, preferably about 11 to 16 days; andloading rates, pretreatment of feed, digester mixing and recycling asknown to the art for anaerobic digestion and pointed out moreparticularly in the references identified above. The present inventionmay be readily applied to multistage digestion, such as exemplified byour earlier U.S. Pat. No. 4,022,665.

An important aspect of the present invention is the anaerobic digestionof biomass consisting of plant and/or organic waste materials ofnormally low biodegradability in the presence of an extract of differentplant material. The plant and/or organic waste material and extract maybe premixed and slurried prior to introduction into the digester or theindividual feed materials may be separately introduced into the digesterand mixed within the digester. The important aspect is that the mixtureof biomass material of normally low biodegradability and extract betogether in the active digestion zone. Feeding and associated wastingmay be continuous or intermittent.

Any active methane producing mesophilic or thermophilic anaerobicdigestion system may be used. Methane-producing anaerobic systemsutilizing acid forming bacteria and methane-producing organisms as arewell known to be employed to produce methane for sewage sludge can beemployed in practice of the present invention. A review of themicrobiology of anaerobic digestion is set forth in AnaerobicDigestion, 1. The Microbiology of Anaerobic Digestion, D. F. Toerien andW. H. J. Hattingh, Water Research, Vol. 3, pages 385-416, Pergamon Press(1969). As set forth in that review, the principal suitablenon-methanogenic bacteria include species from genera includingAerobacter, Aeromonas, Alcaligenes, Bacillus, Bacteroides, Clostridium,Escherichia, Klebsiella, Leptospira, Micrococcus, Neisseria,Paracolobactrum, Proteus, Pseudomonas, Rhodeopseudomonas, Sarcina,Serratia, Streptococcus and Streptomyces. Exemplary methane-producingorganisms suitable for use in the present invention include members ofMethanobacterium, Methanococcus and Methanosarcina, specific membersbeing Methanobacterium formicicum, Methanosarcina barkerii,Methanobacterium omelianskii, Methanococcus vannielii, Methanobacteriumsohngenii, Methanosarcina methanica, Methanococcus mazei,Methanobacterium suboxydans and Methanobacterium propionicum. It isusually preferred to use mixed cultures to obtain the most completefermentation action. Nutritional balance and pH adjustments may be madeto the digester system as is known to the art to optimize methaneproduction from the culture used.

Utilization of normally low biodegradable plant and/or organic wastematerial and extract of a different plant material as a feed for theimproved methane producing process of this invention overcomes priorproblems of seasonable variability of materials for feed stock. Further,storage of plant material feed stocks has not been satisfactory and isexpensive. The use of different plant materials and organic wasteaccording to this invention helps to accommodate the seasonalvariability of various species and geographic locations of farms fortheir production. Utilization of a feed stock including organic wasteprovides simultaneous energy recovery in the form of methane and wastestabilization in an integrated process. The methane containing gasproduced may be treated by methods known to the art to providesubstitute natural gas (SNG).

The process of this invention provides a synergistic yield of methanefrom different plant materials comprising the steps of digesting in anactive mesophilic or thermophilic anaerobic digestion system plant andor organic waste material of normally low biodegradability in thepresence of an extract of different plant material, each present ingreater than an inoculum amount and withdrawing methane-containing gasfrom the digestion system. By methane-containing gas we mean the mixtureof principally methane and carbon dioxide as produced by anaerobicdigestion systems. Various means for increasing methane yield, gasquality and digestion kinetics involving feed pretreatment, residuepost-treatment and recycling or advanced digestion modes may be used inconjunction with the process of this invention.

The following specific examples are set forth for the purpose ofillustration of preferred embodiments and should not limit thisinvention in any way.

EXAMPLE I

Two digesters were operated under the same conditions, the first beingfed only Bermuda grass and the second being fed Bermuda grass plus waterhyacinth extract.

Digester start up was achieved with a mixed inoculum, 70 volume percentderived from an existing mesophilic anaerobic digester fed with sea kelp(Macrocystis pyrifera) and operated at a loading of 0.1 lb. VS/ft³ -dayfor detention time of 18 days and 30 volume percent derived from anotherexisting mesophilic anaerobic digester fed with mixed primary-activatedsewage sludge operated at a loading of 0.8 lb. VS/ft³ -day for detentiontime of 5.6 days. The mixed inoculum contained a diversity of acidforming and methane producing microorganisms as set forth in the Toerienand Hattingh article (Ibid) The digesters were operated with 70 weightpercent sea kelp and 30 weight percent sludge on a VS basis with dailyfeeding and wasting to increase culture volumes by 10% per day to thedesired culture volume of about twice the initial inoculum volume.Loading was maintained at 0.1 lb. VS/ft³ -day and detention time of 15days. Each digester then was passed through a feed transition periodduring which it was fed decreasing amounts of kelp-sludge mixture andincreasing amounts of feed materials as follows:

BERMUDA GRASS DIGESTER

Bermuda grass was prepared by reducing the grass to fine particles byfine extrusion cutting to liberate the cellulose fraction of the fibersfrom the lignin coating. The Bermuda grass was slurried with water forintroduction to the digester.

BERMUDA GRASS+EXTRACT OF WATER HYACINTH DIGESTER

Bermuda grass was prepared as above. Water hyacinth plants were chopped,finely ground and pressed under ambient conditions to result in a liquidextract.

An extract additive was made containing:

97 volume percent water hyacinth extract prepared as above;

1 volume percent activated sludge liquor prepared by centrifugation; and

2 volume percent primary sludge liquor prepared by centrifugation.

The daily feed slurry volume contained 18 volume percent, based upontotal slurry volume, of the above extract additive.

Each digester was operated in a semicontinuous completely mixedanaerobic digestion mode for a detention time of 12 days, a loading of0.1 lb. VS/ft³ -day, and a temperature of 35° C. at a pH of 6.8-7.1. Therun was continued for six detention times and exhibited stableperformance. At steady state, the results of each digester were asfollows:

    ______________________________________                                                     Bermuda   Bermuda grass +                                                     Grass     Extract of Water                                                    Digester  Hyacinth Digester                                      ______________________________________                                        Gas Production Rate                                                           vol/day - vol                                                                 culture        0.240-0.336 0.334-0.544                                        Average        0.304       0.414                                              Gas Yield, SCF/-                                                              lb VS added    2.40-3.47   3.34-5.44                                          Average        3.04        4.16                                               Methane Yield, SCF/-                                                          lb VS added    1.86        2.43                                               ______________________________________                                    

It is seen that gas yield increased about 37 percent and methane yieldincreased about 31 percent by adding 18 volume percent extract of waterhyacinth to the digester. This increase is principally due to increasedmethane and gas production from the Bermuda grass since the theoretical(based upon Volatile Solids content) maximum amount of methane thatcould be produced from the extract would increase methane yield by onlyabout 5%. The digester effluent had very low concentration of solubleorganics and could be dewatered directly by vacuum filtration providingcake-solids content and cake yield comparable to that of filtered,digested sewage sludge.

EXAMPLE II

Two anaerobic digesters were operated with the same feeds as describedin Example I, one Bermuda grass and the other Bermuda grass plus 18volume percent of the daily feed slurry volume water hyacinth extract,as described in Example I, under the same conditions as Example I exceptthe temperature was maintained at 55° C. and loading was 0.5 lb. VS/ft³-day for detention time of 6 days. The results were:

    ______________________________________                                                     Bermuda  Bermuda grass +                                                      Grass    Extract of Water                                                     Digester Hyacinth Digester                                       ______________________________________                                        Gas Production Rate                                                           vol/day - vol                                                                 culture        1.8        2.5                                                 Gas Yield, SCF/-                                                              lb VS added    2.8        4.0                                                 Methane Yield, SCF/-                                                          lb VS added    1.2        2.5                                                 ______________________________________                                    

EXAMPLE III

Two anaerobic digesters are operated under the same conditions asExample I, one with Bermuda grass feed and the other Bermuda grass plug90 volume percent of the daily feed volume water hyacinth extract asdescribed in Example I resulting in the following gas productions:

    ______________________________________                                                     Bermuda  Bermuda grass +                                                      Grass    Extract of Water                                                     Digester Hyacinth Digester                                       ______________________________________                                        Gas Production Rate                                                           vol/day - vol                                                                 culture        0.30       0.50                                                Gas Yield, SCF/-                                                              lb VS added    3.1        5.4                                                 Methane Yield, SCF/-                                                          lb VS added    1.9        3.3                                                 ______________________________________                                    

Methane yield increased about 74% due principally to increased methaneand gas production from the Bermuda grass since the theoretical maximumamount (based upon Volatile Solids content) of methane that could beproduced from the extract would increase methane yield by only about15%.

EXAMPLE IV

Two anaerobic digesters are operated under the same conditions asExample II except for detention time of 4 days. One digester is fed withBermuda grass feed and the other Bermuda grass plus 90 volume percent ofthe daily feed volume water hyacinth extract as described in Example Iresulting in the following gas production:

    ______________________________________                                                     Bermuda  Bermuda grass +                                                      Grass    Extract of Water                                                     Digester Hyacinth Digester                                       ______________________________________                                        Gas Production Rate                                                           vol/day - vol                                                                 culture        1.9        2.6                                                 Gas Yield, SCF/-                                                              lb VS added    2.6        3.8                                                 Methane Yield, SCF/-                                                          lb VS added    1.1        2.2                                                 ______________________________________                                    

Methane yield increased about 100% due principally to increased methaneand gas production from the Bermuda grass since the theoretical maximumamount (based upon Volatile Solids content) of methane that could beproduced from the extract would increase methane yield by only about10%.

EXAMPLE V

Two anaerobic digesters are operated under the same conditions asExample I, one with Bermuda grass feed and the other Bermuda grass plus18 volume percent of the daily feed slurry volume being pure waterhyacinth juice extract (without sludge liquor as in Example I). Theresults are:

    ______________________________________                                                     Bermuda  Bermuda grass +                                                      Grass    Extract of Water                                                     Digester Hyacinth Digester                                       ______________________________________                                        Gas Production Rate                                                           vol/day - vol                                                                 culture        .30        0.45                                                Gas Yield, SCF/-                                                              lb VS added    3.1        4.9                                                 Methane Yield, SCF/-                                                          lb VS added    1.9        3.0                                                 ______________________________________                                    

EXAMPLE VI

Two anaerobic digesters were operated under the same conditions asExample II, one with Bermuda grass feed and the other Bermuda grass plus18 volume percent of the daily feed slurry volume being pure waterhyacinth juice extract (without sludge liquor extract as in Example II).The results are:

    ______________________________________                                                     Bermuda  Bermuda grass +                                                      Grass    Extract of Water                                                     Digester Hyacinth Digester                                       ______________________________________                                        Gas Production Rate                                                           vol/day - vol                                                                 culture        1.8        2.4                                                 Gas Yield, SCF/-                                                              lb VS added    2.8        3.8                                                 Methane Yield, SCF/-                                                          lb VS added    1.2        2.2                                                 ______________________________________                                    

EXAMPLE VII

Two anaerobic digesters were operated under the conditions set forth inExample I with the feed to one being an aqueous slurry of municipalsolid waste and the second a slurry of municipal solid waste in aqueouswater hyacinth extract prepared as described in Example I. The municipalsolid waste and water hyacinth extract were present in equal weightamounts. The municipal solid waste was air separated and reduced to fineorganic-rich particles by two-stage hammermilling. The treated municipalsolid waste was comprised of about 87.4 percent paper and paperproducts; 4.4 percent plastics; 1.3 percent green garbage; and 6.9percent miscellaneous including food waste and paper pieces difficult toidentify, all on a weight percent basis. The results were:

    ______________________________________                                                   Municipal                                                                              Municipal Solid Waste +                                              Solid Waste                                                                            Extract of Water                                                     Digester Hyacinth Digester                                         ______________________________________                                        Gas Production Rate                                                           vol/day - vol                                                                 culture      0.1        0.5                                                   Gas Yield, SCF/-                                                              lb VS added  1.0        6.0                                                   Methane Yield, SCF/-                                                          lb VS added  0.5        3.3                                                   ______________________________________                                    

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for purpose of illustration, it will be apparent tothose skilled in the art that the invention is susceptible to additionalembodiments and that certain of the details described herein can bevaried considerably without departing from the basic principles of theinvention.

We claim:
 1. In a process for improved methane production by anaerobicdigestion, the improvement comprising: prior to said digestion, mixingorganic material of normally low biodegradability, selected from thegroup consisting of plant material, organic waste and mixtures thereof,with an extract of plant material derived from different plant materialthan said low biodegradable plant material to be digested, said extractcomprising about 10 to about 90 volume percent of the digestion volume;anaerobically digesting said mixture for a detention time greater thanabout 4 days; and then removing methane containing gas from thedigester.
 2. The process of claim 1 wherein said extract comprises about15 to about 50 volume percent of said digestion volume.
 3. The processof claim 1 wherein anaerobic digestion is carried out under mesophilictemperatures of about 20° to about 45° C. for detention times of about 8to about 30 days.
 4. The process of claim 1 wherein anaerobic digestionis carried out under thermophilic temperatures of about 45° to about 70°for detention times of about 8 to about 30 days.
 5. The process of claim1 wherein said plane material comprises both terrestrial and aquaticplant materials.
 6. The process of claim 1 wherein said organic materialof normally low biodegradability comprises organic waste.
 7. The processof claim 1 wherein said extract additionally comprises up to about 10volume percent sludge liquor.
 8. The process of claim 7 wherein saidsludge liquor comprises activated and primary sludge liquor.
 9. Theprocess of claim 1 wherein said organic material of normally lowbiodegradability is selected from the group consisting of Bermuda grass,bamboo, Kentucky blue grass, pine trees, poplar trees, eucalyptus,cattails, cornstalks, municipal solid waste and mixtures thereof. 10.The process of claim 1 wherein said extract is derived from plantmaterial selected from the group consisting of water hyacinth, giantbrown kelp, Chlorella, alfalfa, soy bean plants and mixtures thereof.11. In a process of methane production by anaerobic digestion, theimprovement comprising obtaining a synergistic yield of methanecomprising the steps of:prior to said digestion, mixing organic materialof normally low biodegradability, selected from the group consisting ofplant material, organic waste and mixtures thereof, with an extract ofplant material derived from different plant material than said lowbiodegradable plant material to be digested, said extract comprisingabout 10 to about 90 volume percent of the digestion volume; digestingsaid mixture for a detention time greater than about four days in anactive mesophilic or thermophilic anaerobic digestion system; and thenwithdrawing methane-containing gas from said digestion system.